This invention relates to modified carbonate polymer compositions containing polytetrafluoroethylene with and without other additives which act to reduce the susceptibility of such polymer to burning or at least to minimize the effects of dripping of molten polymer when exposed to fire conditions.
Carbonate polymers derived from reactions of dihydroxy organic compounds, particularly the dihydric phenols, and carbonic acid derivatives such as phosgene, have found extensive commercial application because of their excellent physical properties. These thermoplastic polymers are suitable for the manufacture of molded parts wherein impact strength, rigidity, toughness, heat resistance, and excellent electrical properties are required.
Unfortunately, however, these polymers exhibit a brief but definite burning time when contacted with an open flame. More importantly, as is often the case, the carbonate polymers contain stabilizers and other additives which are often more combustible than the unmodified carbonate polymer. As a result, the modified carbonate polymers frequently exhibit sustantially poorer resistance to combustion than do the unmodified carbonate polymers.
In attempts to increase the combustion resistance of carbonate polymers, it has been a common practice to incorporate monomeric phosphites, phosphoric acid esters, thiophosphoric acid esters containing halogenated alkyl radicals, and halogenated organic compounds into the carbonate polymer. However, in order to obtain any noticeable improvement in combustion resistance, these additives have been employed in such large quantities that they often adversely affect many of the desirable physical and mechanical properties of the carbonate polymer.
In U.S. Pat. No. 4,626,563 fire retardant polycarbonates comprising metal salts of sulfimides, chlorinated or brominated aromatic compounds, metal salts of inorganic acids, bisphenol polycarbonates and fibril forming polytetrafluoroethylene (PTFE) are disclosed and claimed. Control 2 therein shows that the addition of 0.5% of a fibril forming polytetrafluoroethylene to the heat stabilized polycarbonate only gave a UL-94 (1/16" thickness) rating of V-2. This is apparently due to the use of more severe extrusion and molding temperatures (302.degree. C.) which destroys the ability of polytetrafluoroethylene to contract upon exposure to the flame and thus the anti-drip characteristic is lost.
U.S. Pat. No. 4,223,100 shows in Table II, Composition 20, a 0.5% PTFE/polycarbonate composition that has a UL-94 (1/8" thickness) rating of V.sub.1 -V.sub.0 and fails the UL-94 test at the 1/16" thickness. Again this is believed to be due to the 300.degree. C. molding temperature.
U.S. Pat. No. 4,110,299 shows in the Table a 0.1% PTFE/polycarbonate composition that fails the UL-94 test due to the 315.degree. C. molding temperature and also low PTFE concentration.